feat(mqtt): schema client-id and write req/resp topic default value

plugins/mqtt/mqtt.json

@@ -6,6 +6,7 @@
     "description_zh": "MQTT 客户端 ID",
     "type": "string",
     "attribute": "required",
+    "default": "neuron_${random_str}",
     "valid": {
       "length": 255
     }
@@ -63,6 +64,7 @@
     "description_zh": "接收点位写入请求的 MQTT 主题。",
     "attribute": "required",
     "type": "string",
+    "default": "/neuron/${random_str}/write/req",
     "valid": {
       "length": 255
     }
@@ -74,6 +76,7 @@
     "description_zh": "发送点位写入响应的 MQTT 主题。",
     "attribute": "required",
     "type": "string",
+    "default": "/neuron/${random_str}/write/resp",
     "valid": {
       "length": 255
     }